This invention relates to a sputtering target formed of refractory metallic silicide and a method of refractory metallic silicide and a method of manufacturing the same and, more particularly, to a sputtering target formed of refractory metallic silicide, having a high density and improved qualities and used for forming thin films as electrodes and wiring members of semiconductor devices, and a method of manufacturing this type of sputtering target.
Conventionally, polysilicon is used for forming electrodes or wiring of semiconductor devices or, more specifically, for forming gate electrodes of metal-oxide-semiconductor (MOS) type large scale integrated circuits (LSI). However, with increase in the degree of integration of LSIs, polysilicon becomes unsatisfactory because of delay of signal propagation due to its resistance which is still disadvantageously large.
On the other hand, for facilitating the formation of devices based on self-alignment, refractory materials to be used as materials for gate, source and drain electrodes are recently in demand, and refractory metallic silicides which are compatible with silicon gate process are regarded as desirable.
Sputtering is known as one of the methods suitable for forming refractory metallic silicide thin films for electrodes or wiring of semiconductor devices. In sputtering methods, argon ions are made to collide against refractory metallic silicide type target so as to release the metal and deposit the same as a thin film on a substrate opposed to the target plate.
The properties of the silicide film formed by sputtering therefore greatly depend upon the characteristics of the target.
Conventionally, such refractory metallic silicide target is obtained by a method (such as the ones disclosed in Japanese Patent Laid-Open Nos. 61-141673 and 61-141674) in which MiSi.sub.2 obtained by reaction between a refractory metal powder (such as a powder of W and Mo) and a silicon (Si) powder is hot-pressed together with Si, or a method (such as the ones disposed in Japanese Patent Laid-Open Nos. 61-58866 and 61-179534) in which a semi-sintered silicide body is impregnated with Si.
These conventional methods, however, entail drawbacks described below. In the case of the former, the proportion of the Si phase ranges from 8 (MSi.sub.2.2) to 25% (MSi.sub.2.9) by volume and is smaller than that of the MSi.sub.2 phase. For this reason, it is not always possible to uniformly spread the Si over the peripheries of angular MSi2 particles obtained by pulverization even if the process is based on hot-press sintering. The resulting target may therefore have a non-uniform and defective structure which has aggregations of angular MSi.sub.2 particles having different sizes, localized Si phase portions and pores and which therefore has a large spatial dispersion of its composition such that the composition varies between central and peripheral portions of the plate-like shape of the target.
The melting point of the MSi.sub.2 phase greatly varies depending upon the kind of refractory metal M. For example, the melting points of WSi.sub.2, MoSi.sub.2, TiSi.sub.2 and TaSi.sub.2 are 2165.degree., 2030.degree., 1540.degree. and 2200.degree. C., respectively. Hot press sintering is effected by using the MSi.sub.2 phase having such different melting points and the Si phase having a melting point of 1414.degree. C. maintained at a temperature not higher than the eutectic temperature. For this reason, sintering cannot be promoted between thermally stable MSi.sub.2 particles, the strength of bonding between the particles is so small that the resulting sintered body is breakable, and pores remain in the resulting structure which reduce the degree of compactness.
In a case where a silicide film is formed by sputtering using a target having such properties, the composition of the silicide film is variable and unstable and it is difficult to obtain the desired film composition, since the distribution of the composition of the target is non-uniform. Also, if sputtering is effected by using a target in which the strength of bonding between particles is small and which has pores, the target may break at its sputtering surface from the positions of defective portions such as those mentioned above so that fine particles come off from the target surface. There is a problem of these particles being mixed in the deposited silicide film.
In a case where work-defect layers having very small cracks or chipped portions formed during mechanical working of the target are not completely removed because target surface finishing is inadequate, or in a case where the load applied through a grinding wheel to the surface to be ground is excessive at the time of grinding of the target so that a residual stress is caused in the target surface portion, the amount of particles coming off from the work-defect layers or residual-stress portions is increased, and the problem of the particles being mixed in the film is also encountered. Other serious problems are thereby experienced, that is, the resistance of the electrode wiring of the resulting semiconductor device is increased or a short-circuit accident takes place, resulting in a considerable reduction in the product yield. Specifically, in the case of high density integrated circuits, the width of the electrode wiring is reduced as the degree of integration is increased as from the 4 megabit scale to the 16 megabit scale, and the short-circuit fraction defective caused by particles mixed in the deposit film is thereby increased abruptly.
In the latter of the above-mentioned conventional methods, the composition of the target is controlled by the density of the semi-sintered silicide body. However, in a case where a semi-sintered body having a predetermined density is made by synthesis of MSi.sub.2 based on silicide reaction between a metal powder and a silicon powder, or in a case where a semi-sintered body having a predetermined density is made by sintering a pressed compact formed from an MSi.sub.2 powder, the density varies depending upon the processing temperature, the processing time and the pressing pressure, and it is therefore very difficult to obtain a target having the desired composition.
Further, according to the inventors' knowledge, there is no possibility of impurities being collected by diffusion at the interfaces between the MSi.sub.2 phase and the Si phase of the target because high-purity MSi.sub.2 and Si products are ordinarily used as raw-material powders, and the strengths of interface bonding between the MSi.sub.2 phase and the Si phase and interface bonding between MSi.sub.2 phase portions are therefore small.
There is also a problem of the sputtering operation being unstable because the difference of the electrical resistances between the MSi.sub.2 phase and the Si phase is extremely large. That is, the electrical resistances of WSi.sub.2, MoSi.sub.2, TiSi.sub.2 and TaSi.sub.2 capable of constituting the MSi.sub.2 phase are 70, 100, 16, and 45.mu..OMEGA..cm, respectively, and are thus small, while that of the Si phase is 2.3.times.10.sup.10 .mu..OMEGA..cm and is thus extremely large. The electrical resistance therefore abruptly changes at the interface between the MSi.sub.2 phase and the Si phase, also because no impurity diffusion layers exist at the interfaces. Specifically, in the structure of the target made by the latter method, the Si phase constituting the matrix phase is in a continuous form, and small-resistance MSi.sub.2 phase portions are surrounded by the large-resistance Si phase portion.
If sputtering is effected by using a target having such a structure, insulation breakdown of the MSi.sub.2 phase and the Si phase necessarily takes place at a voltage higher than a certain level, and a discharge starts flowing. That is, if the applied voltage is higher than a certain level, discharge takes place, and parts of MSi.sub.2 particles or the Si phase of a small interface strength thereby come off and disperse as fine particles.
Serious problems are thereby encountered; the resistance of the electrode wiring of the resulting semiconductor device is increased or a short-circuit accident takes place, resulting in a considerable reduction in the product yield.
If sputtering is effected under optimum deposition conditions by using a conventional target formed of an alloy of a refractory metal and silicon to form a refractory metallic silicide film on a substrate, the distribution of the film thickness can be made uniform but the proportion of the refractory metal in the film composition is larger at a central portion of the substrate than at a peripheral portion, and a trough-curve distribution of the MSi.sub.x composition is exhibited.
Accordingly, the distribution of the sheet resistance of the thin film is as represented by a trough curve, and the dispersion of the electrical characteristics in the film on the substrate surface is large.
It is considered that this non-uniformity of the distribution of the composition of the thin film formed on the substrate surface is mainly due to a large difference between the sputter release angle distributions of the refractory metal (M) and the silicon (Si) and due to the variation of the amount of Si atoms dispersed by argon (Ar) atoms which variation depends upon the relative positions of the target and the substrate.
In addition, since the film composition at the substrate center is rich in the refractory metal, the film stress thereat is high and the thin film may be easily peeled off during heating oxidation in a later processing step.
It is difficult to prevent occurrence of such non-uniformity of the film composition even by variously changing the deposition conditions relating to the position of the magnetic field generator and the processing voltage.
The inventors had an idea of forming a target having a composition distribution reverse to the film composition distribution such that the distribution of the composition of a thin film formed by sputtering using this target can be made uniform.
In the conventional manufacturing methods, however, it is extremely difficult to obtain a target having a composition distribution in which the proportion of a component is continuously changed and, hence, to make the composition distribution in the formed thin film uniform.